The downstream regulatory element of the proU operon of Salmonella typhimurium inhibits open complex formation by RNA polymerase at a distance

The intracellular concentration of K+-glutamate, chromatin-associated prote ins, and a downstream regulatory element (DRE) overlapping with the coding sequence, have been implicated in the regulation of the proU operon of Salm onella typhimurium, The basal expression of the proU operon is low, but it is rapidly induced when the bacteria are grown in media of high osmolarity (e.g. 0.3 M NaCl). It has previously been suggested that increased intracel lular concentrations of K+-glutamate activate the proU promoter in response to increased extracellular osmolarity. We show here that the activation of the proU promoter by K+-glutamate in vitro is nonspecific, and the in vivo regulation cannot simply be mimicked in vitro. In vivo specificity require s both the chromatin-associated protein H-NS and the DRE; they are both nee ded to maintain repression of proU expression at low osmolarity, How H-NS a nd the DRE repress the proU promoter in vivo has so far been unclear. We sh ow that, in vivo, the DRE acts at a distance to inhibit open complex format ion at the proU promoter.